On museum tours, plant tours, and the like, it is useful to have a series of pre-recorded, taped messages to be listened to by the visitor. If these messages come from a speaker located at each station of the exhibit, visitors may have difficulty in hearing it because of ambient noise or interference from another speaker.
Accordingly, visitors may carry a pre-tuned radio receiver to receive a weak broadcast signal. In this way, recorded messages are automatically heard by visitors in the vicinity, distracting noises are avoided, and the visitor can spend as much or as little time as he wishes at each station. Transmission to the receiver is normally from an induction loop proximate to the listening zone.
Such systems have inherent problems of "cross-talk" between adjacent broadcasting zones in which both stations are heard at the same time. One way of preventing "cross-talk" is to keep the power level of either the carrier wave or the amplifier in the receiver low or to separate the antennas; this, however, caused difficulties for the hard of hearing. Another solution has been to use multiple carrier frequencies, using different ones for each zone. This has, in the past, required a more expensive receiver adapted for several frequencies and operation of the receiver by the visitor. Another system has been to include a scanner which selects the strongest carrier frequency and eliminates others; this is complex and expensive.
The phenomenon of interfering modulated AM signals has been previously observed. See Terman, Electronic and Radio Engineering, 1955, p. 565; Terman, Radio Engineers' Handbood, 1943, p. 577; Aiken, Theory of Detection of Two Modulated Waves by a Linear Rectifier, Proc. I.R.E., Vol. 21, April 1933, p. 601, and the references cited. The phenomenon, however, appears to have been viewed as a problem, having no utility, and creating, whistle, sideband noise, or flutter. For it to effectively suppress the weaker signal, the ratio between the signal amplitudes supposedly should exceed 2:1. Apparently no practical use of the phenomenon, such as is the present system, has been suggested.